leetcode 669. Trim a Binary Search Tree

Given a binary search tree and the lowest and highest boundaries as L and R, trim the tree so that all its elements lies in [L, R] (R >= L). You might need to change the root of the tree, so the result should return the new root of the trimmed binary search tree.

Example 1:

Input: 
    1
   / \
  0   2

  L = 1
  R = 2

Output: 
    1
      \
       2

Example 2:

Input: 
    3
   / \
  0   4
   \
    2
   /
  1

  L = 1
  R = 3

Output: 
      3
     / 
   2   
  /
 1

  解法1:

凡是树的题目无非DFS，BFS，递归或者迭代做，仅此而已。

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def trimBST(self, root, L, R):
        """
        :type root: TreeNode
        :type L: int
        :type R: int
        :rtype: TreeNode
        """
        # use DFS, root first then left and right
        # if L <= root.val <= R: 
        #    new_root with root.val; 
        #    new_root.left = self.trimBST(root.left, L, root.val)
        #    new_root.right = self.trimBST(root.right, root.val, R)
        #    return new_root
        # elif root.val < L:
        #    return self.trimBST(root.right, L, R)
        # elif root.val > R:
        #    return self.trimBST(root.left, L, R)
        
        if root is None:
            return None
        if L <= root.val <= R:
            new_root = TreeNode(root.val)
            new_root.left = self.trimBST(root.left, L, root.val)
            new_root.right = self.trimBST(root.right, root.val, R)
            return new_root
        elif root.val < L:
            return self.trimBST(root.right, L, R)
        elif root.val > R:
            return self.trimBST(root.left, L, R)

迭代解法：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode trimBST(TreeNode root, int L, int R) {
        if (root == null) {
            return root;
        }
        while (root.val < L || root.val > R) {
            if (root.val < L) {
                root = root.right;
            }
            if (root.val > R) {
                root = root.left;
            }
        }
        TreeNode dummy = root;
        while (dummy != null) {
            while (dummy.left != null && dummy.left.val < L) {
                dummy.left = dummy.left.right;
            }
            dummy = dummy.left;
        }
        dummy = root;
        while (dummy != null) {
            while (dummy.right != null && dummy.right.val > R) {
                dummy.right = dummy.right.left;
            }
            dummy = dummy.right;
        }
        return root;
    }
}